In a typical digital data system arranged for point-to-point data transfer over a digital network there is a local data terminal equipment (DTE) and a local digital data unit (DDU) connected to a remote DDU and a remote DTE via a digital network. DTE are devices such as terminals and personal computers serve as an interface between a user and the DDU. The DDUs combine the functions of a data service unit (DSU) and a channel service unit (CSU) into a single unit which makes it possible to transfer data between a local and remote site over the digital network. The digital network is supplied by a service provider and may operate at various data rates, such as 64 kilobits per second and greater. DDUs that are available for transferring data over the digital network include a Motorola DDS/MR64 and similar devices.
In order to maintain and establish data connections, testing methods or modes have become an integral part of DDUs. Most of the various test modes are initiated and terminated via a control lead (or connection) from the DTE to the DDU or from a front panel switch located on the DDU. One of these test, the remote loop back test, serves as a tool to assist maintenance personnel at a local site to isolate and detect trouble conditions in a digital data system. The remote loop back test may verify operation of the local DTE, the local DDU, the digital network, and the remote DDU.
When a command to perform the remote loop back test is initiated by the local DTE, the local DDU will send a test signal to the remote DDU, causing it to enter the remote loop back test mode. When the remote DDU is in the loop back test mode, data from the local DTE or other test data transmitted by a local transmitter in the local DDU will be received by the remote DDU and looped back through a remote receiver and remote transmitter (within the remote DDU) and returned to a local receiver in the local DDU. If the transmitted test data and the data returned to the local receiver are the same (except for a time delay), then the local DDU, the digital network, the remote DDU and corresponding connectors are functioning properly and the remote loop back test is successful.
Presently there are two recognized methods of initiating and terminating the remote loop back test. Each method initiates the test by transmitting a test (or protocol) signal, from the local DDU, to the remote DDU. The test signals (or sequences) for both methods use a pseudo random pattern (or sequence) generated by a scrambler using a scrambler polynomial equal to 1+X-4+X-7. The scrambler, using the scrambler polynomial, may be implemented using a seven stage shift register with feedback taken from the forth and seventh stages. When the input sequence to the scrambler is a binary zero sequence (a continuous string of zeros) the pseudo random pattern produced is a repeating pattern, 127 bits in length, generally known as the PN127 sequence. When the input to the scrambler is a binary one sequence (a continuous string of ones), a scrambled binary ones sequence is generated which is sometimes known as the inverse PN127 sequence. The scrambled binary ones sequence may also be generated by inverting the PN127 sequence. Scramblers are well known to those in the art and are discussed in textbooks such as Digital Transmission Systems by David R. Smith (1985) and are used in a variety of data communications products designed according to certain standards, such as Recommendations V.34 and V.54 of the International Telecommunications Union (ITU).
The first of the recognized methods, a prior art approach, to perform the remote loop back test uses the procedures described in the ITU (previously known as the Consultative Committee on International Telephony and Telegraphy or CCITT) Recommendation V.54. Recommendation V.54 requires the local DDU to continuously transmit (via the local transmitter) the PN127 sequence until at least 2048 bits are transmitted. The at least 2048 bit sequence transmitted in accordance with V.54 is called a preparatory pattern (or sequence). The local DDU having sent the preparatory pattern will then search, using a local receiver, for an acknowledge pattern from the remote DDU. When the local receiver detects the acknowledge pattern the remote loop back test is successful. Recommendation V.54 defines the acknowledge pattern to be at least 1948 bits of the scrambled binary ones sequence. In order to comply with V.54, the remote DDU, upon recognition of the preparatory pattern, must transmit the acknowledge pattern and then enter the remote loop back test mode. The signal for termination of the remote loop back test is initiated by the local DDU and requires the local transmitter to transmit a termination pattern, comprising 8192 bits of scrambled binary ones followed by 64 binary ones, to the remote DDU.
The second prior art method to perform a remote-loop back test uses a technique (or protocol) jointly developed by American Telephone and Telegraph (AT&T) and U.S. Sprint (sometimes called the Sprint method). In the second method, the local DDU transmits the PN127 sequence for a period of two seconds, then sends binary ones for an additional 2 seconds. The remote unit, upon recognition of the PN127 sequence for a period of 256 error free milliseconds, enters the remote loop back test mode. When the local receiver detects reflected data (data returned from the remote DDU) then the remote loop back test is successful. The termination of the remote loop back test is initiated by the local DDU when the local transmitter transmits the scrambled binary ones sequence for 2 seconds followed by binary ones for an additional 2 seconds.
A local DDU designed according to the first method, i.e., Recommendation V.54, will not be compatible with a remote DDU designed according to the Sprint method when attempting to initiate and complete the remote loop back test. Because each of these recognized methods have advantages and disadvantages both are widely used in digital data systems. Hence some DDUs are designed to operate according to the V.54 protocol, while others are designed to operate according to the Sprint protocol. The differences in the two methods may cause incompatibility problems when DDUs are coupled via the digital network and are required to perform the remote loop back test.
Accordingly, a need has remained to reliably initiate and perform a remote loop back test from a local digital data unit when the test method (or protocol) of a remote digital data unit is unknown. In addition, a need has remained to implement such a test efficiently, with minimal complexity and with a minimal response time, and with equipment having a lower or reduced cost to manufacture or purchase.